There have been several reports of efforts to use interferometric schemes in microscopy. Laser homodyne and heterodyne interferometry can measure extremely small displacement changes. In a scanning microscopy application this should allow detailed mapping of surfaces with ultra-fine depth resolution.
Matthews et al. in Applied Optics, 2372 (1986) and T. Suzuki et al. in Applied Optics, 3623 (1991) have demonstrated the potential of optical homodyne interferometry in 3-D microscopy. The optical pathlength of the reference arm of the interferometer in these experiments was actively controlled to maintain the interferometer at maximum phase sensitivity. The error signal from a feedback control loop was monitored while scanning the surface to obtain its 3-D structure. Unfortunately, this method is extremely sensitive to environmental disturbances; i.e. any thermal and acoustic change in optical path length of the interferometer arms can disturb the measurement. Huang was able to overcome these difficulties by the use of a double beam heterodyne interferometer with common-mode rejection that allowed a depth resolution of 10 picometers. An optical heterodyne interferometric scheme was proposed by See et al. in Applied Optics, p. 2373 (1985). In their experiment the probe beam was periodically displaced at 1.7 MHz and the phase of the probe beam was modulated by depth variations. These depth variations were converted into intensity contrast on a cathode ray tube display. In this way they were able to identify the grain boundaries on a polished stainless steel surface. Although they achieved phase sensitivity close to the photon noise limit, because of the wide noise bandwidth (30 KHz) and small probe power (50 .mu.W), the maximum differential depth resolution obtained was approximately 0.01 nm. Moreover, because the amplitude of the displacement was approximately the size of the focus, the spatial resolution was relatively poor compared to other microscopy techniques. In the instant invention below, a more versatile and high resolution microscopy technique is introduced that uses a true heterodyne interferometry which uses direct phase locked loop RF demodulation that narrows the bandwidth of the measurement, which gives almost two orders of magnitude improvement over the average depth resolution when compared to the concept introduced by See et al. above.
The most pertinent art cited as to the heterodyne interferometry technique of the instant invention includes Fujita et al. U.S. Pat. No. 4,995,726 entitled "Surface Profile Measuring Device Utilizing Optical Heterodyne Interference" and Fujita et al. U.S. Pat. No. 5,061,071 entitled "Method and Apparatus for Detecting Surface Characteristics by Utilizing Optical Heterodyning Interferometry" where the sample being measured, is oscillated by a piezoelectric device in a vertical mode parallel to the optical axis. Differences between the instant invention and these references include the mode of oscillation by the oscillating driver means in a lateral mode versus the vertical mode as taught by the prior art. The instant inventions apparatus and method is much less complex then these prior art references resulting in greater reliability. Moreover, the instant invention uses a geometrical average path-length methodology for determining displacement in the surface which can use a homodyne demodulation technique if required which is not discussed in Fujita et al.